This invention relates to a system for sensing the phase sequence of a three-phase AC power supply and for connecting a phase-sensitive, three-phase load to that power supply in a manner dictated by the phase sequence sensed in order that the AC power, as applied to the load, will have a desired sequence.
The invention may be employed in any three-phase electrical system where a phase-sensitive load is employed, such as a motor-driven compressor for a refrigeration system. Unless the phase voltages are applied to the motor in the correct sequence, the motor will rotate in the wrong direction and the compressor will not function properly. The invention is particularly attractive in an environment where the AC power source must be energized and requires a warm-up or stabilizing period during which the line voltages slowly build up to normal magnitude and, moreover, where the AC supply provides phase voltages having either one of the two possible phase sequences. For example, this may be the case when an engine generator (such as a diesel generator) is employed as the three-phase AC power source. Hence, the invention is ideally applicable to a refrigeration system in a truck.
The present invention provides a novel phase sequence correcting system for insuring that the phase voltages applied to a three-phase load are correctly sequenced. This is achieved by means of a unique digital logic arrangement which is of relatively inexpensive construction; requires very little power; is highly efficient, accurate and reliable; and can be contained within a small space.